二叉树的层次遍历,lintcode

给出一棵二叉树，返回其节点值的层次遍历（逐层从左往右访问）

样例

给一棵二叉树 {3,9,20,#,#,15,7} ：

3

/ \

9 20

/ \

15 7

返回他的分层遍历结果：

[

[3],

[9,20],

[15,7]

]

解题思路：用一个队列实现，队列保存每层的节点，以此遍历每层。

一刷ac

/**
* Definition of TreeNode:
* public class TreeNode {
*     public int val;
*     public TreeNode left, right;
*     public TreeNode(int val) {
*         this.val = val;
*         this.left = this.right = null;
*     }
* }
*/

public class Solution {
/**
* @param root: The root of binary tree.
* @return: Level order a list of lists of integer
*/
public ArrayList<ArrayList<Integer>> levelOrder(TreeNode root) {
ArrayList<ArrayList<Integer>> rst = new ArrayList<ArrayList<Integer>>();
if(root == null) return rst;
LinkedList<TreeNode> list = new LinkedList<TreeNode>();
list.offer(root);
while(!list.isEmpty()){
ArrayList<Integer> res = new ArrayList<Integer>();
int size = list.size();
for(int i = 0; i < size; i++){
TreeNode node = list.poll();
if(node.left != null) list.offer(node.left);
if(node.right != null) list.offer(node.right);
res.add(node.val);
}
rst.add(res);
}
return rst;
}
}